1. Field of the Invention
The present invention relates to an air-intake duct and an air-intake structure for guiding air to a throttle device coupled to an engine.
2. Description of the Related Art
An engine mounted in a motorcycle and other vehicles includes a cylinder head having a combustion chamber. An air-intake structure forming an air-intake passage is coupled to an intake port of the combustion chamber to guide air and fuel to the combustion chamber. The air-intake structure typically includes an air cleaner box of an air cleaner, an air-intake duct, and a throttle body of a throttle device which are coupled to each other in this order from an upstream side in an air flow direction. The throttle body is provided with an injector for injecting a fuel.
Japanese Laid-Open Patent Application Publication No. 2006-90298 discloses a double-injector air-intake structure applied to a high power engine, in which an injector (upstream injector) is provided inside an air cleaner box in addition to the above injector (downstream injector), and an air inlet of an air-intake duct is disposed to face an injection port of the upstream injector so that air cleaned by the air cleaner and fuel injected from the upstream injector are efficiently guided to the throttle device.
The air-intake duct forming the double-injector air-intake structure has a coupling function for coupling in an air tight manner the air cleaner box to the throttle body and an air guiding function for guiding the cleaned air and the fuel to the throttle device. In the conventional air-intake structure disclosed in the above Publication, a coupling member for performing the coupling function and an air guiding member for performing the air guiding function are integrally formed of the same material, and therefore it is difficult to perform these functions in a well-balanced manner.
For example, the coupling member is desirably formed of an elastic rubber material to ensure air-tightness. If both the coupling member and the air guiding member are formed of the elastic rubber material, it is necessary to increase the wall thickness of the air guiding member to maintain its shape. This narrows an air passage and degrades the air guiding function. In addition, the weight of the air-intake duct increases because of an increase in a wall thickness of the air guiding member, thereby resulting in a reduced fuel efficiency. On the other hand, if both the coupling member and the air guiding member are formed of a material (synthetic resin, metal, etc) other than the elastic rubber material, then a seal member such as an O-ring is needed to ensure air-tightness in the coupling member. This reduces a mounting efficiency of the air-intake duct.
If the coupling member and the air guiding member are molded integrally using a die, the entire air-intake duct has a complex shape. For this reason, undercut frequently occurs, design flexibility is lessened, and manufacturing cost increases because of complexity of the die.
The air-intake performance of the air-intake passage can be controlled precisely by changing the length of the air guiding member. To this end, in the conventional structure, it is necessary to replace the entire air-intake duct. This task is burdensome. Therefore, it is not easy to control the air-intake performance of the air-intake passage.